| Summary: | 碩士 === 國立中興大學 === 昆蟲學系 === 89 === Carbohydrate metabolism and its possible regulation in larvae of the common armyworm, Spodoptera litura, in response to different physiological conditions were studied. Under normal feeding condition, the concentration of carbohydrate (i.e. blood sugar) in the fifth instar larvae of S. litura , maintained at about 2 mg/ml and began to increase before head capsule slippage. In contrast, the levels during the sixth instar increased gradually and reached about 15-16 mg/ml before pupation. The blood sugar levels of the sixth instar larvae increased significantly within 5 h of starvation and ligation by 25 and 42%, respectively; furthermore, the activity of glycogen phosphorylase in the fat body increased significantly by 24% within 5 h of starvation and by 34% within 2 h of ligation. Nevertheless, the blood sugar concentrations increased significantly within 9 h of ligation, while the percentage of active glycogen phosphorylase in the fat body did not significantly increase. In contrast to the blood sugar, the levels of blood lipid in hemolymph of the sixth instar larvae stayed low under starvation, suggesting that the blood sugar, but not blood lipid, was main energy source for the sixth instar larvae.
The blood sugar levels increased by 2.6 fold when refed the 24-h-starved sixth instar larvae for 1 h and then decreased after 2 h, suggesting that there may be a regulatory system to maintain a constant blood sugar level. When refed the 4-h and 8-h-starved sixth instar larvae for 1 h, the fat body glycogen phosphorylase activity was significantly decreased by 16 and 25%, respectively, however, there were no significant changes in the blood sugar concentrations. This inactivation of the enzyme action could also be obtained by injection of 3 mg glucose into the 4-h-starved sixth instar larvae, while injection of 3 mg trehalose was less effective. These results indicate that glucose in the hemolymph may play a role in the regulation of fat body glycogen phosphorylase activity.
Neck-ligation increased the blood sugar levels and the glycogen phosphorylase activity in the fat body, suggesting that there may be hypoglycemic factor(s) in the heads. However, the blood sugar concentrations were increased significantly by 28% in 4-h-ligated sixth instar larvae after injection of crude head extracts (2 head equivalents) for 30 min, indicating that there were hyperglycemic factor(s) in head. Additionally, the head extract treatments did not affect the glycogen phosphorylase activity in the fat body, suggesting that the increase in the blood sugar levels was not via the hydrolysis of the fat body glycogen. Moreover, injection of 10 mmole octopamine for 30 min caused significant increase of 26% blood sugar levels and 31% glycogen phosphorylase activity in the fat body, suggesting that ligation may cause the release of octopamine from nerves to induce hyperglycemia.
In conclusion, the results in this study suggest that the starvation stimulates the elevation of blood sugar concentrations and glycogen phosphorylase activity in the fat body of S. litura larvae，it is probably cuased by a decrease of glucose in the hemolymph. Hyperglycemic factor(s) in the head and stress-induced release of octopamine may manage the supply of carbohydrates during starvation.
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